The development of high energy battery systems requires, among other things, the compatibility of an electrolyte solution possessing desirable electrochemical properties with respect to highly reactive anode materials, such as lithium or the like. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has, therefore, been necessary in order to realize the high energy density obtainable through use of these highly reactive anodes to turn to the investigation of nonaqueous electrolyte systems.
The term "nonaqueous electrolyte" as used herein refers to an electrolyte which is composed of a solute, such as, for example, a metal salt or a complex salt of Group IA, Group IIA, Group IIIA or Group VA elements of the Periodic Table, dissolved in an appropriate nonaqueous solvent. The term "Periodic Table" as used herein refers to the Periodic Table of Elements as set forth on the inside front cover of the Handbook of Chemistry and Physics, 63rd Edition, CRC Press Inc., Boca Raton, Fla., 1982-1983.
A multitude of solutes is known and many have been suggested for use but the selection of a suitable solvent has been particularly troublesome. The ideal battery electrolyte would comprise a solvent-solute pair which has a long liquid range, high ionic conductivity and stability. A long range, i.e., high boiling point and low freezing point, is essential if the battery is to operate at other than normal ambient temperatures. High ionic conductivity is necessary if the battery is to have high rate capability. Stability is necessary with the electrode materials, the materials of cell construction, and the products of the cell reaction to provide long shelf life when used in a primary or secondary battery system.
It has been disclosed in the literature that certain materials are capable of acting both as an electrolyte carrier, i.e., as solvent for the electrolyte salt, and as the active cathode for a nonaqueous electrochemical cell. U.S. Pat. Nos. 3,475,226, 3,567,515 and 3,578,500 each disclose that liquid sulfur dioxide or solutions of sulfur dioxide and a cosolvent will perform this dual function in nonaqueous electrochemical cells. While these solutions perform their dual function, they are not without several disadvantages in use. An amount of sulfur dioxide is always present and, being a gas at ordinary temperatures, it must be contained in the cell as a liquid under pressure or dissolved in a liquid solvent.
U.S. Pat. No. 4,400,453 discloses a nonaqueous electrochemical cell comprising an anode, a cathode collector and a cathode-electrolyte, said cathode-electrolyte comprising a solution of an ionically conductive solute dissolved in an active cathode (depolarizer) wherein said active cathode (depolarizer) consists of a liquid oxyhalide of an element of Group V or Group VI of the Periodic Table. Although oxyhalides can be used effectively as a component part of a cathode-electrolyte in conjunction with an active metal anode, such as a lithium anode, to produce a good high energy density cell, it has been observed that if the cell is stored for a period of about three days or longer, excessive passivation of the anode appears to occur which results in undesirable voltage delays at the beginning of discharge along with high cell impedance.
U.S. Pat. No. 3,993,501 discloses one approach for minimizing or preventing undesirable voltage delays at the beginning of discharge of nonaqueous cells employing an oxyhalide-containing cathode-electrolyte by providing a vinyl polymer film coating on the surface of the anode that contacts the cathode-electrolyte.
U.S. Pat. No. 4,218,523 discloses a nonaqueous cell comprising an active metal anode, such as lithium, a liquid cathode-electrolyte comprising a solute dissolved in a solvent which is an oxyhalide of an element of Group V or Group VI of the Periodic Table and wherein elemental sulfur or a sulfur compound is incorporated into the cathode-electrolyte so as to substantially eliminate initial voltage delay of the cell during discharge.
U.S. Pat. No. 4,277,545 discloses a nonaqueous cell utilizing an active metal anode, such as lithium, a cathode collector and an ionically conductive cathode-electrolyte comprising a solute dissolved in a liquid cathode, such as an oxyhalide, and wherein a vinyl polymer is dissolved in the cathode-electrolyte so as to substantially eliminate initial voltage delay of the cell during discharge.
U.S. Pat. No. 4,020,240 discloses an electrochemical cell employing an electrolyte salt containing a clovoborate anion structure which functions to retard anode passivation during long time storage even at elevated temperatures.
U.S. Pat. No. 4,071,664 discloses an electrochemical cell comprising an active metal anode and an electrolyte solvent/cathode depolarizer which reduces anode passivation during long time storage, even at elevated temperatures, by the inclusion of a minor proportion of an electrolyte salt additive having a clovoborate anion structure.
U.S. Pat. No. 4,228,229 discloses a nonaqueous lithium/oxyhalide cell in which the passivation of the lithium is effectively eliminated by employing a solute for the electrolyte solution which includes a complex salt resulting from the reaction of at least one ionizable compound with aluminum chloride, and wherein said ionizable compound comprises lithium sulphide Li.sub.2 S, lithium oxide Li.sub.2 O, calcium oxide CaO, or barium oxide BaO.
U.S. Pat. No. 4,440,836 discloses a nonaqueous cell employing an active metal anode, such as lithium, having a surface layer of a boron-containing material, a cathode and an electrolyte solution containing a solute dissolved in a nonaqueous solvent such as an oxyhalide in which case the oxyhalide also acts as the active cathode and whereby the surface layer of the boron-containing material reduces the initial voltage delay of said cell.
One of the objects of the present invention is to reduce the initial voltage delay of a nonaqueous cell employing a cathode-electrolyte solution comprising a solute dissolved in a liquid active cathode.
Another object of the present invention is to prevent the excessive passivation of an active metal anode in a liquid cathode-electrolyte cell by adding a boron-containing additive to the cell.
Another object of the present invention is to prevent the excessive passivation of an active metal anode in a liquid cathode-electrolyte cell by adding a boron-containing compound and a vinyl polymer to the cell, said boron-containing compound and vinyl polymer additives providing a synergistic effect in reducing the initial voltage delay of said cell.
Another object of the present invention is to provide an oxyhalide cathode-electrolyte cell system employing an active metal anode, such as lithium, with a boron-containing compound additive and a vinyl polymer additive to effectively prevent the excessive passivation of the active metal anode during cell storage and usage.
The foregoing and additional objects will become more fully apparent from the following description.